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January 19, 2012

It's been a long time coming; Dsavitsk started looking for builders for his prototype of the Torpedo, a CCS loaded, transformer coupled parafeed headphone amplifier back in March of 2010. The Torpedo is a simplified and more affordable version of the ECP AudioL'espressivo spud headphone amplifier and works with 6J6 tubes and equivalents, including the 5964 and 5844. The shortkit was just recently made available in December 2011 on Beezar after a number of DIY enthusiasts built the initial prototype and includes the chassis, custom transfomers from Edcor and the PCB. The remainder of the parts are listed in the bill of materials and can be purchased from the usual suspects (Mouser, Digikey, etc.). The total build cost, if not going crazy, should fall under $300.

The Torpedo Schematic from ECP Audio

The ECP Audio / Beezar Torpedo Shortkit

The most unique facet of the Torpedo is the length. At 14" deep, it's a very unique looking unit. The length was designed to keep the output transformers as far away from the power transformer as possible to minimize potential flux interaction. As anyone who's built a headphone amplifier with output transformers can tell you, this is a real concern; noise is very apparent when a sensitive driver is right next to your ears.

In the interest of a unique looking build, an antique copper / bright copper powdercoat color scheme was selected. Holes were drilled every inch in a line along the top left and top right for copper pop-rivets to embellish the chassis. The rear vent was cut out in favor of copper colored perforated metal above the power transformer. Also, holes were added to the bottom of the case for EAR isolation feet.

Fancy powdercoated Torpedo case (Bottom and Top)

The EAR feet were installed with a combination of a #6 screw and washer on the bottom and a lock washer and nut on top to ensure that they remain screwed in.

Case bottom with EAR isolation feet

The initial population starts with the smaller parts, resistors, diodes, etc. This build won't deviate much from the bill of materials as the specified parts are of high quality, however the obligatory Kiwame carbon film resistors were dropped in. Since Kiwame / Koa Speers doesn't offer their resistors in 1M values, Takman carbon films were used in their stead. Also, the Neutrik 1/4" jack was substituted with the gold-plated version as their isn't much of a price difference.

Initial population of the Torpedo Board

A matched pair of 4.7μF Clarity Cap ESAs were used as the parafeed caps. The ESAs are the same size as the SAs but with slightly nicer specs and tighter tolerances. The price was right at about $34 for the pair as of this writing and they're a perfect fit. The purpose of these caps is to block direct current (DC) from the output transformers.

Clarity Cap ESAs at 4.7uF

Almost fully populated

According to TomB and Dsavitsk, there is a small bit of audible noise when the top of the case is fitted on a completed Torpedo amp. Unfortunately at this point, builders have not been able to isolate the issue, despite trying things like grounding the heatsinks and adding grounded shielding to try to isolate the power transformer from the tubes. This build will be trying a slightly different approach; the tubes will be raised up out of the chassis using a custom fabricated metal panel that the tubes will mount to, hopefully protecting them from the power transformer noise. The PCB will be wired to a pair of chassis mount tube sockets which will sit on the metal panel raised by a set of nylon standoffs. The ideal measurement for the standoffs seems to be about 40mm, or a hair above 1.5 inches.

Below is the metal panel designed in Front Panel Express that Dsavitsk and I worked out. There are three holes to attach the nylon standoffs and two holes for the 7 pin tube sockets and the accompanying chassis mounts. This will necessitate two holes being drilled into the PCB to accommodate the left and right standoffs, which will be used to ensure a rigid mount so tubes can be swapped without the panel flexing.

Tube mounting panel designed in Front Panel Express

Below is the FPE fabricated panel in their nice bronze anodized color.

Tube socket mounting board

Unfortunately, the panel was a little too wide at 39mm, so it was trimmed down a bit with a cutting disc. Also, underneath one of the screw holes are the traces on the PCB for the left and right channel inputs; we wouldn't want to knock those out, so a new screw hole was made a little closer to the tube socket on that side. Since the screws are so close to the top of the chassis, they were depth mounted to avoid any clearance issues. Finally, the ceramic tube sockets had to be sanded ever-so-slightly to fit in the holes.

Tube sockets mounted

Now came the time to wire up the tube socket board. Because some of the pins are interconnected on the PCB, they would be shorted on the socket to decrease the amount of wiring necessary (1+2) and (5+6). A hundred ohm resistor was used to connect pins 5+6 to the PCB. This was done at the advice of Dsavitsk. Also added was a twisted pair of wires coming from pins 3 and 4 (the heater section) that will be powering an LED-based pilot light. Kimber TCSS wiring was used, it's my preferred hook up wire in tube amps as it's relatively thick (19ga) but easy to work with and has a Teflon coating. The pins of the socket were covered with heatshrink tubing to prevent shorting.

Tube socket board wired up

Holes were drilled into the PCB (into the ground plane area) and the nylon standoffs were mounted. The wires were carefully trimmed to the right size and stripped at the end and tinned (covered with solder) so that the wire strands wouldn't stray when inserted into the PCB holes. The board was screwed to the nylon standoffs. Finally, the amp is fully populated.

Fully populated Torpedo amp

Wiring detail under the mounting board

When the cover is added, the sockets come right up to the top of unit, perfect for rolling tubes without having to unscrew anything.

Sockets once the cover has been mounted

The amp assembly can now be completed. The pilot light with bayonet style LED was wired up, the front panel holes were tapped for 4-40 screws and screws installed, the grounding screw and standoffs secured, and a Kilo aluminum knob installed on the Alps Blue Velvet potentiometer.

Completed Torpedo Amplifier

Completed Torpedo - Back

Completed Torpedo - Front

Initial testing was done with a pair of headphones with 32ohm impedance. There is a smidge of audible noise that sounds like it's from flux interaction, but it's barely audible and hardly worth mentioning. I'd definitely consider it a non-issue. The amp sounds excellent. Highly recommended!

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The Fine Print:

Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.

January 3, 2012

People feel so strongly about audio reproduction that it often becomes akin to religion and politics and arguments ensue. Some listeners feel that a perfectly flat response curve and fully diminished THD levels (total harmonic distortion) are the holy grail, while there are others that enjoy the warm distortion and harmonics achieved with old triode technology and a frequency response that tapers the upper and lower extremes. Both are right in their own way, the former looking for perfect accuracy, the latter looking for a relaxing listening experience, and there are probably many listeners that fall in between.

The Objective2 headphone amp falls into the first category, where good measurements are the paramount objective. NwAvGuy has gone to great lengths to provide what many headphone listeners have been pining for, a low-cost headphone amp that achieves great measurements in a vast number of categories. While I don't fall into the category of pursuing perfect measurements (just look at all the number of tubes used in the builds on this site), I highly respect the effort that went into this build and all subsequent documentation.

Something that's quite impressive is that NwAvGuy does not make a penny from these designs. The PCB files are open source and circulated freely; users are encouraged to arrange group-buys on forums such as DIYAudio.com in order to procure the board and/or the front panel. Also, as of this writing, JDSLabs is offering both the O2 board and a unique front panel (to see the JDSLabs CMoyBB being built, please see my previous post). The total cost of the build is very reasonable and should fall under $100 including the case, custom front panel and wall-wart. Add a couple hours of soldering time and you'll have yourself an amp.

The Objective2 Sept 2011 PCB board

What's especially important with closely populated boards is starting with the small parts (resistors and diodes) and working your way up to the larger items such as capacitors and MOSFETs. If one doesn't follow this basic principal, it's much more difficult to fit your fingers into the small recesses between the larger items on the board.

Partially populated, all resistors and small parts in place

The battery clips may look crooked, but they were lined up carefully before soldering to ensure a perfect for for a 9V. This is something one doesn't want to try and adjust later as desoldering them can be difficult. The faceplate shown was procured from a DIYAudio group buy. Many thanks to MrSlim and FlynHawaiian for their efforts.

Fully populated board with faceplate

Faceplate from the DIYAudio group buy

There are a number of testing procedures on NwAvGuy's site to ensure the amp is working properly before you plug a pair of headphones in to ensure that you don't inadvertently damage them with high DC output.

A pair of Tenergy 9V rechargables were dropped in and secured with double sided tape and the unit was boxed up with the case specified by the bill of materials, a Box Enclosures B2-080. Gold was chosen as it's a bit more flashy than a plain black box. Screws were replaced with brass counterparts and the knob was a generic aluminum one from the parts bin, possibly made by Kilo.

Assembled Objective 2 amp

Once the amp had a few minutes to burn in, some cans were plugged in for a listen. All online reviews of this amp were avoided so the initial listening could be done without any pre-conceived notions. The amp reminds me quite a bit of the headphone output of the Benchmark DAC from a HeadFi meet. It's very crisp, clear and has strong, effortless bass. The bass may even be a bit better than the Benchmark was, if memory serves correctly. Both amps are quite analytical; I wouldn't choose them for my own personal musical enjoyment as my ears are sensitive and need the treble and upper-midrange to be a bit rolled off or fatigue sets in too quickly. One can see this amp being highly recommended for audio mastering work as it should convey every detail possible to the editor.

Immediately after building the amp, the input and output jacks were shorting with a variety of different plugs. It appears I received a bad batch as I didn't see any other documentation online regarding issues with the Kycon jacks. Rather than replace them with the identical items, Switchcraft jacks will be wired in as they are very trustworthy. The opamps will also be switched to the low-power versions made by Texas Instruments, which hopefully will be a little bit warmer.

The Fine Print:

Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.